The present invention relates to an ion beam machining apparatus for directly machining a very small portion of an electronic part, such as a semiconductor or the like, and, more particularly, to an ion beam machining apparatus which is capable of high speed machining by using a projection ion beam.
In this technical field, there is a direct machining technology utilizing sputtering produced by irradiating a Focused Ion Beam (FIB) on a sample, and there is a method of observing a section of a semiconductor device or modifying wirings by using the machining technology (Japanese Unexamined Patent Publication No. 02-295040).
As is well known, a FIB is formed by focusing an ion beam emitted from a liquid metal ion source onto a sample using an electrostatic lens system to thereby produce an image of the ion source. The image of the ion source is very small and, therefore, there is provided a beam diameter determined mainly by aberration of the electrostatic lens system, and the size of which practically falls in a range of several 10 nm through several xcexcum. Further, in the application thereof to direct machining, generally the acceleration voltage is set to 20 through 60 kV and the current falls in a range of several pA through 10 nA. The FIB can be irradiated to an arbitrary point and scanned in a region of about 1 mm at maximum on a sample by using an electrostatic deflector. In this case, in the case of using a FIB, due to the aberration of the electrostatic lens, when the current is increased, the beam diameter is enlarged. When the beam current reaches several nA, blur is rapidly enhanced due to the spherical aberration of the electrostatic lens and the current density is reduced. Therefore, the beam diameter is switched by changing the aperture or the focused state in accordance with the machining area and machining accuracy such that the machining speed is reduced as little as possible. Particularly, in forming a cross section of a sample for observation by a scanning electron microscope (SEM), as shown in FIG. 7(a), which is most generally carried out by machining using a FIB, a hole having a rectangular region of about 10 xcexcm is dug to a similar degree of depth and machining is carried out by switching the beam to a slender beam successively twice through three times in order to finish only a face for observing the cross section. Further, a similar operation is carried out in forming a wall of a sample for observation by a transmission electron microscope (TEM), as shown in FIG. 7(b).
Further, there is known projection ion beam technology capable of forming a pattern with high accuracy. According to Japanese Unexamined Patent Publication No. 2-65117, the beam is used in lithography and an area having a size of several 10 mm is exposed by an accuracy of sub xcexcm. It is regarded that, although the projection ion beam is suitable for irradiating a large area with a high accuracy, the beam is not suitable for an application which needs a high current density to carry out direct sputtering machining. In this case, a technology for carrying out high accuracy machining by applying an FIB apparatus to a projection ion beam apparatus is disclosed in Japanese Unexamined Patent Publication No. 8-213350. In accordance with this technology, the beam current is 10 nA at most, similar to that in a FIB, since the constitution of the FIB apparatus is used and there is no description with regard to a technology enabling high speed machining which can replace the FIB.
As described above, there is no known ion beam forming technology capable of realizing a machining speed exceeding that of a FIB and which is capable of forming a region of several 10 xcexcm or smaller with an accuracy of sub xcexcm.
It is an object of the present invention to provide a projection ion beam machining apparatus which is capable of machining a region having a size of several 10 xcexcm or smaller, at high speed, and of processing an edge of the region with high accuracy by using an ion beam projecting a pattern of a member having an opening portion (stencil mask).
The present invention is based on optimum conditions in the design of an electrostatic lens system and indispensable matters in constituting an apparatus which we have found for constituting a projection ion beam apparatus capable of machining at high speed and with high accuracy in comparison with a FIB. The optimum conditions in the design of an electrostatic lens system referred to here are mainly optimum ranges of a distance between an ion source and a lens proximate to the ion source, a distance between a sample and a lens proximate to the sample and a distance between these two lenses. To satisfy such design conditions, specific combinations with regard to the constitution of the apparatus are needed.
An explanation will be given of conditions in the design of an electrostatic lens system. First, for simplicity an investigation has been made of a case in which two electrostatic lenses, that is, a lens 1 proximate to an ion source and a lens 2 proximate to a sample are used, as shown in FIG. 8(a) and FIG. 8(b). FIG. 8(a) shows a case of forming a FIB in which the intensities (inverse number of focal length) of the two lenses are adjusted such that an image of the ion source is formed on the sample by the ion beam. FIG. 8(b) shows a case of forming a projection ion beam in which the strength of the lens 2 is adjusted such that an image of a stencil mask is formed on the sample by the ion beam. In this case, the lens 1 is an illumination lens for adjusting the amount of irradiating ions which impinge onto the stencil mask, and the lens 1 converges the ion beam to a center of the lens 2. Further, the lens 2 is a projection lens for projecting the image of the mask onto the sample, and the lens 2 converges the ion beam radiated from respective points of the stencil mask onto the sample along trajectories shown as dashed lines in FIG. 8(b).
The following has been found by investigating characteristics of the two ion optical systems by calculation. That is, when the distance between the ion source and the center of the lens 1 is designated by notation Lo, the distance between the sample and the center of the lens 2 is designated by notation Li and the distance between the centers of the lens 1 and the lens 2 is designated by notation L, in the case of FIG. 8(a), the current density of the FIB on the sample is proportional to 1/(Loxc3x97Li). In the meantime, in the case of FIG. 8(b), the current density of the projection ion beam on the sample is proportional to the square of L/(Loxc3x97Li). That is, when a comparison is made with the same lens arrangement, a ratio of the current density of the projection ion beam as compared with the current density of the FIB is proportional to (L/Lo)(L/Li). Further, in the case of the projection ion beam, when the current density is increased, distortion is increased in proportion to the ninth power of Lo and the third power of Li even in a pattern having the same size and the same current density.
It has been found from the foregoing results that in the electrostatic lens system of the projection ion beam apparatus according to the present invention, it is necessary to increase L and reduce Lo and Li, and more particularly to reduce Lo to minimize the distortion to a degree which is not conceivable in the case of a FIB apparatus. For such purpose, it is indispensable to arrange all of the elements of the ion optical system, other than the electrostatic lenses, between the electrostatic lenses. In the meantime, when L is increased, the accuracy of the setting voltage of a lens power source necessary for adjusting the strength (inverse number of focal length) of the electrostatic lens proximate to the ion source becomes more and more severe and, accordingly, it has been found that there is an upper limit for L. However, it has been also found that this restriction can be alleviated and L can be effectively increased when the electrostatic lenses are increased in three stages. Further, it has been found that a focusing condition of the ion beam for minimizing the distortion of the projected pattern on the sample differs for each size of the pattern of the mask. Therefore, it has been found that there is needed a mechanism capable of switching the intensities of the respective electrostatic lenses in cooperation with the size of the pattern of the mask. The above-described discussion assumes that the ion beam passes on the central axes of the lenses, and it has been found that when the ion beam is disposed off of the axes (particularly in the lens proximate to the sample), the position of the projection ion beam on the sample is shifted and an edge of the pattern is significantly distorted. Hence, it has been found that a deflector for always accurately guiding the ion beam on the axes is indispensable. Further, it has been found that when the strength of the lens for projecting the mask is inaccurate, the size of the pattern of the ion beam on the sample is varied. Hence, it has been found that in order to confirm the condition of the projection lens, means for effectively moving the ion source, that is, a deflector arranged on the ion source side of the mask, is indispensable.
Specifically, the problem is solved by a projection ion beam machining apparatus provided with an ion source, a stage for holding a sample, a first electrostatic lens disposed between the ion source and the stage and provided on the side of the ion source, a second electrostatic lens provided on the side of the stage, a mask having an opening portion provided between the first electrostatic lens and the second electrostatic lens, a first electrostatic deflector provided between the mask and tile first electrostatic lens and two stages of electrostatic deflectors provided between the second electrostatic lens and the mask.
Further, in the above-described projection ion beam machining apparatus, the first electrostatic lens is a lens synthesized by arranging an acceleration lens having two electrodes and an Einzel lens having three electrodes (lens in which potentials of the electrodes at both ends are the same) successively from the side of the ion source.
Furthermore, in the above-described projection ion beam machining apparatus, an aperture is arranged between the acceleration lens having the three electrodes and the Einzel lens having the three electrodes.
Furthermore, in the above-described projection ion beam machining apparatus, the second electrostatic lens is an Einzel lens.
Furthermore, the above-described projection ion beam machining apparatus has an electrostatic deflector for blanking the ion beam on the side of the ion source of the second electrostatic lens and a fixed aperture arranged on the side of the sample of the electrostatic deflector.
Furthermore, in the above-described projection ion beam machining apparatus, the ion source is a liquid metal ion source.
Furthermore, in the above-described projection ion beam machining apparatus, the sample is machined with a high sputtering efficiency by making the acceleration voltage of the ion beam equal to or higher than 20 kV and equal to or lower than 60 kV.
Further, in the above-described projection ion beam machining apparatus, the mask is provided with a group of a plurality of selectable openings and a control system for controlling operation of the electrostatic lens system is provided with means for storing two sets or more of control parameters and means for changing the sets of the control parameters for respective ones of the openings of the mask.
Furthermore, in the above-described projection ion beam machining apparatus, at least one of the openings of the mask is formed in a circular shape.
Further, the above-described projection ion beam machining apparatus has means for detecting secondary particles (secondary elections, secondary ions or secondary beam) emitted from the sample, means for scanning the ion beams on the sample by using the two stages of electrostatic deflectors provided between the second electrostatic lens and the mask or the electrostatic deflector provided between the mask and the first electrostatic lens and means for forming and displaying an image of the sample by using an output signal provided from the means for detecting the secondary particles in synchronism with the scanning.
Furthermore, the above-described projection ion beam machining apparatus has means for designating a position for irradiating the projection ion beam onto the sample by using a display of a one-dimensional or two-dimensional image of the sample.
Furthermore, the problem is solved by a projection ion beam machining apparatus which is provided with a liquid metal ion source, a stage for holding a sample, a combination of two or three electrostatic lenses arranged between the ion source and the sample and a mask having opening portions disposed in the combination of the electrostatic lenses, wherein, when the distance between substantial center of the electrostatic lens most proximate to the ion source and an ion emitting portion of the ion source is designated by notation Lo, the distance between a substantial centers of the electrostatic lens most proximate to the sample and a surface of the sample is designated by notation Li and the distance between the substantial centers of the two lenses is designated by notation L, a value of (L/Lo)(L/Li) is equal to or larger than 400.
Further, the problem is solved by a projection ion beam machining apparatus which is provided with a liquid metal ion source, a stage for holding a sample, a combination of two or three electrostatic lenses arranged between the ion source and the sample and a mask having opening portions arranged in the combination of the electrostatic lenses, wherein the distance between an ion emitting portion of the ion source to a surface of the sample is equal to or larger than 400 mm and is equal to or smaller than 1500 mm, the distance from the ion source to an end of a side of the sample of the electrostatic lens most proximate to the ion source is equal to or smaller than 40 mm and the distance from an end of a side of the ion source of the electrostatic lens most proximate to the sample to a surface of the sample is equal to or smaller than 40 mm.
Further, the problem is solved by a projection ion beam machining apparatus which is provided with an electrostatic lens system for forming an ion beam for projection an opening pattern of a mask having an opening portion onto a sample held by a sample stage, wherein the current density of the ion beam on the sample is equal or larger than 20 mA per one square cm.
Furthermore, in the above-described projection ion beam machining apparatus, the edge resolution power, when the size of the ion beam on the sample is 5 xcexcm, is equal to or smaller than 0.2 xcexcm.